The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
In the field of an automatic transmission, more multiplicity of shift-stages is used to enhance fuel efficiency and drivability of a vehicle. The increase of oil price makes it difficult to improve fuel consumption of a vehicle.
The engine downsizing has been attempted for purposes of weight reduction and fuel efficiency. Simultaneously, the development of more shift stages of an automatic transmission has caused to provide the enhanced driving stability and fuel efficiency.
For more shift-stages of an automatic transmission, the number of parts is typically increased, which may affect installability, production cost, weight and/or power flow efficiency.
In that aspect, less number of parts is desired to enhance fuel efficiency of an automatic transmission having more shift-stages.
An eight-speed automatic transmission has been introduced recently and a planetary gear train for an automatic transmission enabling more shift-stages is under development.
An automatic transmission of eight or more shift-stages typically includes three to four planetary gear sets and five to seven control elements (frictional elements), and this may largely affect the weight of the vehicle.
In this regard, disposing planetary gear sets in parallel or employing dog clutches instead of wet-type control elements has been attempted. However, such an arrangement may not be widely applicable, and using dog clutches may easily deteriorate shift-feel.